In the field of light-absorbing filters, the technique of incorporating a dye into a constituent layer for the purpose of absorbing a light having a specific wavelength has generally been employed extensively. Further, dyeing with dyes each having a specific absorption spectrum is being extensively conducted for the purpose of producing pulp products having various color tones. The dyes used for such purposes should not only have spectral characteristics suitable for the intended use, but also satisfy requirements, e.g., to have a high absorbance index, to have excellent stability with freedom from a concentration change or discoloration with the lapse of time in dyed products or in dye solutions, and not to be discolored or faded by the influence of other additives. Moreover, the dyes for use in producing these products preferably are dyes which have sufficient solubility in solvents therefor and do not cause decoloring after dyeing.
Many investigations have been made so far in order to satisfy the requirements described above. For example, a merocyanine dye was proposed in U.S. Pat. No. 3,615,608, benzylidene dyes were proposed in U.S. Pat. Nos. 3,540,887 and 3,544,325 and JP-B-31-10578 (the term "JP-B" as used herein means an "examined Japanese patent publication"), a styryl dye was proposed in U.S. Pat. No. 1,845,404, an oxonol dye was proposed in British Patent 506,385, and a hemioxonol dye was proposed in U.S. Pat. No. 3,652,289. Although some of these dyes have an appropriate absorption spectrum, the others are known to have a drawback that they undergo considerable fading or an undesirable change in absorption spectrum due to the influence of the product pH or of other additives. In addition, many of those proposed dyes either have low solubility and hence hardly give a dye solution having a concentration necessary for giving an absorption intensity required of a product, or have insufficient dyeing power even though having high solubility. Thus, few dyes satisfy all of the requirements described above.
A merocyanine dye having both a benzothiazole ring and a rhodanine ring as terminal auxochromes is disclosed in, e.g., JP-B-54-34331. However, there has been no known merocyanine dye which has a rhodanine ring substituted with a carboxyalkyl group containing an ester, amide, or ether bond within the substituent.